International Patent Publication WO 98/07913 discloses a device for controlling warp threads for the production of leno fabrics on a textile machine. Preferably the known device is part of a weaving loom and includes a sley carrying a conventional reed. A needle carrier carrying a multitude of needles is positioned above the fabric plane on the side of the reed facing the warps. The fabric plane is determined by the position of the beat-up line which extends in the fabric plane. Each of the needles comprises a needle eye. The needle carrier is operatively connected with a drive which oscillates the needle carrier together with the leno warp threads in a motion extending vertically to the fabric or weaving plane for opening and closing the shed.
A first heald frame is arranged downstream of the needle carrier on the warp facing side of the reed. The first heald frame is driven in a plane vertically to the weaving or fabric plane. The first heald frame comprises a guide bar positioned approximately in the fabric plane. The guide bar is provided with a multitude of slots arranged in a row and extending at a slant. One warp leno thread is guided in each of these slots, whereby an oscillatory cross motion is imposed on the warp threads in response to the oscillating vertical motion of the heald shaft. A second heald shaft is positioned on the warp side downstream of the first heald shaft. The second heald shaft is driven in a plane vertically to the weaving plane. The second heald shaft comprises a rod positioned approximately in the weaving plane. The rod is equipped with a slot extending crosswise and parallel to the weaving plane. The leno warp threads are guided in this slot.
A third heald frame drive for a vertical oscillating movement is equipped with a pair of compensating rollers. These rollers are driven relative to the third heald frame and the rotational direction of these rollers can be reversed in a controlled manner, for example by a drive that is operatively connected with a drive shaft.
The above described conventional warp thread control requires a substantial effort and expense for its construction due to the numerous components and their intricate cooperation. Another disadvantage is seen in that at least three separate drives are required, namely one drive for operating the heald frames, a second drive for the needle bar, and a third drive for the compensation rollers.
Another disadvantage of the conventional warp thread control is seen in that it requires means referred to as so-called negatively effective elements for displacing the leno threads in case the leno threads are insufficiently tensioned or if they are exposed to uneven tensions. In that case the leno threads are no longer positively guided in the slanted slots of the heald or shaft frame. As a result at least some of the leno threads do not perform an oscillating horizontally extending motion relative to the ground threads. Hence, faulty bindings and flaws are unavoidably formed in the leno fabric.